My impression is that they do not know those kind of details, what they get is basically the same application you can print off ERAS. That being said, I know in many instances there is friendly chat that happens amongst PD's in different fields, and I guess it is plausible that the same applicant's name is mentioned, albeit pretty unlikely. In regards to how many interviews you get, none of them know this (I have been asked though) but I think that an experienced PD can pretty much guess if you've gotten a lot or only a few.
But then again, what do I know, I'm only a med student.